The report claims that Venezuelan officials gave Farc rebels a safe haven, using them to train pro-government miiitias. Photograph: Scott Dalton/AP

Venezuela has attacked a report which links President Hugo Chavez's government to Colombian rebels as a "dodgy dossier".

The Venezuelan embassy in London accused the International Institute for Strategic Studies of recycling unfounded allegations in a report published on Tuesday.

The 240-page analysis, based on thousands of intercepted documents of the Revolutionary Armed Forces of Colombia, details a close and at times tempestuous relationship between the guerrillas and Chavez's government.

Titled The FARC Files: Venezuela, Ecuador and the Secret Archive of "Raúl Reyes", the report says Venezuelan officials afforded the rebels a safe haven, used them to train pro-government militias and asked them to assassinate opponents.

The study, with an accompanying CD of documents, contradicts Chavez's claim that only a few rogue members of his government collaborated with the cocaine-funded, Marxist-tinged insurgency in neighbouring Colombia.

The IISS report is based on electronic files recovered during a March 2008 raid by Colombian forces on a Farc camp inside Ecuador which killed a senior rebel commander, Raúl Reyes, and yielded a trove of computers, hard discs and memory sticks.

The Venezuelan embassy issued a swift statement branding the report "a Latin America dodgy dossier" which contained inaccuracies and appeared to be "part of an aggressive propaganda tool" against Caracas.

The embassy said Colombia's supreme court had ruled as inadmissible material from the computers in prosecution cases against Colombian politicians. It also said Interpol had faulted Colombian authorities' handling of the documents.

"The embassy has said that international community should not accept as valid the use of unverified files by the British institute."

After examining the files in 2008 Interpol said Colombian authorities had not conformed to "internationally recognised principles" in handling electronic evidence but that the archives had not been doctored.

Colombia has shared some of the material with Canada, Spain and the US, among other governments, and several institutions and journalists. The IISS report, said to be two years in the compiling, appears to be the most detailed analysis yet.

It depicts a fraught alliance between Chavez's socialist government and the rebels who challenge Colombia's government and its US-backed armed forces. Venezuela's president has tailored his support according to political need, said the report. "Whenever he has judged it expedient, he has been ready to put the relationship on the back burner and even act against Farc interests."

It describes a clandestine meeting between in Venezuela in 2000 in which Chavez allegedly agreed to lend Reyes cash to buy arms. Two years later Venezuela's president allegedly betrayed the rebels and handed over eight Farc operatives to gain an edge in a meeting with Colombia's then president, Alvaro Uribe.

According to the dossier Mono Jojoy, a Farc leader, called Chavez a "deceitful and divisive president who lacked the resolve to organise himself politically and militarily".

The relationship recovered and Venezuela's intelligence agency, then known as Disip, allegedly asked Farc to train Venezuelan state security forces and militias in urban warfare and guerrilla tactics.

In 2006 Venezuelan officials asked the Farc to assassinate Henry Lopez Sisco, a former intelligence chief from a previous government, according to a document attributed to Reyes. "They ask that if possible we give it to this guy in the head."

Lopez Sisco left Venezuela later that year and there is no evidence any attempt was made on his life, nor that Chavez authorised any assassination attempts.

In recent months Chavez has mended relations with Colombia and moved against the Farc. Colombia's government, which is keen to nurture the detente, said it had no comment on the IISS report.